Apparatus for determining the position and orientation of an invasive portion of a probe inside a three-dimensional body

ABSTRACT

System for sensing at least two points on an object for determining the position and orientation of the object relative to another object. Two light emitters mounted in spaced relation to each other on an external portion of an invasive probe remaining outside an object into which an invasive tip is inserted are sequentially strobed to emit light. Three light sensors or detectors, the positions of which are known with respect to a predetermined coordinate system, detect the positions of the two light emitters positioned on the probe. A computer connected to the probe and to the light sensors receives data from the sensors and determines the position and orientation of the probe relative to the predetermined coordinate system. The computer then determines the position and orientation of the invasive portion of the probe inside the object by correlating the position of the invasive portion of the probe relative to the predetermined coordinate system with the position of a model of the object defined in relation to the predetermined coordinate system. A display device connected to the computer indicates the location of the invasive portion of the probe in the object by displaying a representation of the location of the invasive portion of the probe with respect to the model of the object.

This is a continuation of U.S. Ser. No. 08/052,042, now abandoned whichis a continuation-in-part of U.S. Ser. No. 07/909,097, U.S. Pat. No.5,383,454, which is in turn a continuation-in-part of U.S. Ser. No.07/600,753, now abandoned, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved method and apparatus fordetermining, in real time, the position of the tip of an invasive probeinside a three-dimensional object and displaying its position relativeto a geometrical model of that object visually displayed on a computerscreen. More specifically, this invention relates to an improved methodand apparatus of interactively determining the position of a probe tipinside the head of a patient during intracranial surgery relative to athree-dimensional internal diagnostic image of that patient.

2. Description of the Prior Art

Computed tomography (CT), magnetic resonance imaging (MRI), and othermethods provide important detailed internal diagnostic images of humanmedical patients. However, during surgery there often is no obvious,clear-cut relationship between points of interest in the diagnosticimages and the corresponding points on the actual patient. Whileanomalous tissue may be obviously distinct from normal healthy tissue inthe images, the difference may not be as visible in the patient on theoperating table. Furthermore, in intracranial surgery, the region ofinterest may not always be accessible to direct view. Thus, there existsa need for apparatus to help a surgeon relate locations in thediagnostic images to the corresponding locations in the actual anatomyand vice versa.

The related prior art can be divided into art which is similar to thepresent invention as a whole and art which is related to individualcomponents of this invention.

Prior art similar to the present invention as a whole includes methodsof correlating three-dimensional internal medical images of a patientwith the corresponding actual physical locations on the patient in theoperating room during surgery. U.S. Pat. No. 4,791,934 does describe asemi-automated system which does that, but it requires additionalradiographic imaging in the operating room at the time of surgery as themeans to correlate the coordinate systems of the diagnostic image andthe live patient. Furthermore, the system uses a computer-driven robotarm to position a surgical tool. In particular, it does not display thelocation of an input probe positioned interactively by the surgeon.

There have been other attempts to solve the three-dimensionallocalization problem specifically for stereotactic surgery. One class ofsolutions has been a variety of mechanical frames, holders, orprotractors for surgery (usually intracranial surgery). For examples seeU.S. Pat. Nos. 4,931,056; 4,875,478; 4,841,967; 4,809,694; 4,805,615;4,723,544; 4,706,665; 4,651,732; and 4,638,798. Generally, these patentsare intended to reproduce angles derived from the analysis of internalimages, and most require rigidly screwing a stereotactic frame to theskull. In any case, these methods are all inconvenient, time-consuming,and prone to human error.

A more interactive method uses undesirable fluoroscopy in the operatingroom to help guide surgical tools (U.S. Pat. No. 4,750,487).

More relevant prior art discloses a system built specifically forstereotactic surgery and is discussed in the following reference:

David W. Roberts, M.D., et al; "A Frameless Stereotaxic Integration ofComputerized Tomographic Imaging and the Operating Microscope", J.Neurosurgery 65, October 1986.

It reports how a sonic three-dimensional digitizer was used to track theposition and orientation of the field of view of a surgical microscope.Superimposed on the view in the microscope was the correspondinginternal planar slice of a previously obtained computed tomographic (CT)image. The major disadvantages reported about this system were theinaccuracy and instability of the sonic mensuration apparatus.

Although the present invention does not comprise the imaging apparatusused to generate the internal three-dimensional image or model of thehuman patient or other object, the invention does input the data fromsuch an apparatus. Such an imaging device might be a computed tomography(CT) or magnetic resonance (MRI) imager. The invention inputs the datain an electronic digital format from such an imager over a conventionalcommunication network or through magnetic tape or disk media.

The following description concentrates on the prior art relatedspecifically to the localizing device, which measures the position ofthe manual probe and which is a major component of this invention.Previous methods and devices have been utilized to sense the position ofa probe or object in three-dimensional space, and employ one of variousmensuration methods.

Numerous three-dimensional mensuration methods project a thin beam or aplane of light onto an object and optically sense where the lightintersects the object. Examples of simple distance rangefinding devicesusing this general approach are described in U.S. Pat. Nos. 4,660,970;4,701,049; 4,705,395; 4,709,156; 4,733,969; 4,743,770; 4,753,528;4,761,072; 4,764,016; 4,782,239; and 4,825,091. Examples of inventionsusing a plane of light to sense an object's shape include U.S. Pat. Nos.4,821,200, 4,701,047, 4,705,401, 4,737,032, 4,745,290, 4,794,262,4,821,200, 4,743,771, and 4,822,163. In the latter, the accuracy of thesurface sample points is usually limited by the typically low resolutionof the two-dimensional sensors usually employed (currently about 1 partin 512 for a solid state video camera). Furthermore, these devices donot support the capability to detect the location and orientation of amanually held probe for identifying specific points. Additionally,because of line-of-sight limitations, these devices are generallyuseless for locating a point within recesses, which is necessary forintracranial surgery.

The internal imaging devices themselves (such as computed tomography,magnetic resonance, or ultrasonic imaging) are unsuited for tracking thespatial location of the manually held probe even though they areunencumbered by line-of-sight restrictions.

A few other methods and apparatus relate to the present invention. Theytrack the position of one or more specific moveable points inthree-dimensional space. The moveable points are generally representedby small radiating emitters which move relative to fixed positionsensors. Some methods interchange the roles of the emitters and sensors.The typical forms of radiation are light (U.S. Pat. No. 4,836,778),sound (U.S. Pat. No. 3,821,469), and magnetic fields (U.S. Pat. No.3,983,474). Other methods include clumsy mechanical arms or cables (U.S.Pat. No. 4,779,212). Some electro-optical approaches use a pair of videocameras plus a computer to calculate the position of homologous pointsin a pair of stereographic video images (for example, U.S. Pat. Nos.4,836,778 and 4,829,373). The points of interest may be passivereflectors or flashing light emitters. The latter simplify finding,distinguishing, and calculating the points.

Probes with a pointing tip and sonic localizing emitters on them havebeen publicly marketed for several years. The present invention alsoutilizes a stylus, but it employs tiny light emitters, not soundemitters, and the method of sensing their positions is different.

Additional prior art related to this patent is found in thesereferences:

Fuchs, H.; Duran, J.; Johnson, B.; "Acquisition and 10 Modeling of HumanBody Form Data", Proc. SPIE, vol. 166, 1978, pp. 94-102.

Mesqui, F.; Kaeser, F.; Fischer, P.; "Real-time, Non-invasive Recordingand 3-D Display of the Functional Movements of an Arbitrary MandiblePoint", SPIE Biostereometrics, Vol. 602, 1985, pp. 77-84.

Yamashita, Y.; Suzuki, N.; Oshima, M. "Three-Dimensional StereometricMeasurement System Using Optical Scanners, Cylindrical Lenses, and LineSensors", Proc. SPIE, vol. 361, 1983, pp. 67-73.

The paper by Fuchs, et al., (1978) best describes the method used by thepresent invention to track the surgical probe in three-dimensionalspace. It is based on using three or more one-dimensional sensors, eachcomprising a cylindrical lens and a linear array of photodetectors suchas a charge-coupled semiconductor device (CCD) or a differential-voltageposition sensitive detector (PSD).

The sensors determine intersecting planes which all contain a singleradiating light emitter. Calculation of the point of intersection of theplanes gives the location of the emitter. The calculation is based onthe locations, orientations, and other details concerning theone-dimensional sensors and is a straightforward application of analyticgeometry. This electro-optical method, however, has not been previouslyused for the purpose of the present invention.

Thus, there still remains a need for a complete apparatus which providesfast, accurate, safe, convenient mensuration of the three-dimensionalposition of a manual probe and which visually relates that position tothe corresponding position on the image of a previously-generatedthree-dimensional model of an object.

SUMMARY OF THE INVENTION

A first objective of the present invention is to provide accurate,three-dimensional mensuration of the location and orientation of aninstrument on or inside an object, which could be (but is not limitedto) a surgical patient in an operating room.

A second objective of this invention is to provide an electro-opticalmensuration system which is inexpensive, easy to use, reliable, andportable and which employs a manually positioned probe or other pointinginstrument.

A third objective of this invention is to provide a simple, non-invasivemeans of establishing a correspondence between a predeterminedcoordinate system of the object and a coordinate system of athree-dimensional, geometrical computer model of that object where thecomputer model has been provided as input data to this invention.

A fourth objective of this invention is to relate a measured location onor inside the object to the corresponding location in the computer modelof that object according to the established correspondence between thecoordinate systems of the object and the model.

A fifth objective of this invention is to display a cut-away view or across-sectional slice of that model on the graphics screen of theinvention, where the slice may be a planar cross-section of thegeometrical model, where the slice approximately intersects the locationin the model corresponding to the measured location. A marker may thenbe superimposed on the displaced slice to indicate the location on theslice corresponding to the measured location.

A sixth objective of this invention, is specifically to help a surgeonlocate diseased tissue while avoiding healthy critical structures,especially in cranial neurosurgery.

Additional objects, advantages, and novel features of the inventionshall be set forth in part in the following description and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by the practice of the invention. Theobjects and the advantages of the invention may be realized and attainedby means of the instrumentalities and in combinations particularlypointed out in the appended claims.

To achieve the foregoing and other objects and in accordance with theinvention, as embodied and broadly described herein, the opticalmensuration and correlation apparatus comprises a hand held probe havingan invasive tip for touching or for inserting into an object. Two ormore light emitters mounted in spaced relation to each other on theexternal portion of the probe remaining outside the object aresequentially strobed to emit light. Three or more light sensors ordetectors, the positions of which are known with respect to apredetermined coordinate system, detect the positions of the two or morelight emitters positioned on the probe as they are strobed. Acomputer-coupled to the probe and to the light sensors receives datafrom the sensors and calculates the position and orientation of theprobe, with respect to the predetermined coordinate system. The computerthen determines the position and orientation of the invasive portion ofthe probe inside the object by correlating the position of the invasiveportion of the probe relative to the predetermined coordinate systemwith a model of the object defined relative to the predeterminedcoordinate system. A display device coupled to the computer indicatesthe location of the invasive portion of the probe in the object bydisplaying a representation of the location of the invasive portion ofthe probe with respect to the model of the object.

The method of this invention includes the steps of detecting theposition of the probe relative to the predetermined coordinate system,computing the position and orientation of the invasive portion of theprobe relative to the predetermined coordinate system, determining theposition and orientation of the invasive portion of the probe inside theobject by correlating the position of the invasive portion of the proberelative to the predetermined coordinate system with the model of theobject, and indicating the location of the invasive portion of the probein the object by displaying a representation of the location of theinvasive portion of the probe with respect to the model of the object.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing figures, illustrate a preferred embodiment ofthe present invention and, together with the description, serve toexplain the principles of the invention.

FIG. 1A-B are block diagrams of the optical mensuration and correlationapparatus of the present invention showing the major components.

FIG. 2 is a perspective drawing illustrating the invention in use by asurgeon performing intracranial surgery on a patient, and showing acursor on the display screen that marks the corresponding position ofthe invasive tip of the probe within the image of previously obtainedmodel data.

FIG. 3 is a sample of the display showing a position of tip of the probewith respect to previously obtained model data and showing the referencepoints on the patient's skull display as triangles.

FIG. 4 is a schematic perspective representation of one of theone-dimensional photodetectors of the present invention.

FIG. 5 is a graph of the image intensity (manifested as a voltage orcurrent) versus locations on the photodetector surface for a typicallight detector used by the optical mensuration and correlation apparatusof the present invention.

FIGS. 6 and 7 are diagrams of the major steps performed by the computerto calculate the position of the invasive portion of the probe withrespect to the model of the object and to display the image slice.

FIG. 8 is an illustration of the manner in which the threeone-dimensional measurements determine three intersecting planesintersecting at a uniquely determined point.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The optical mensuration and correlation apparatus 10 of the presentinvention is shown schematically in FIGS. 1A-B and comprises a hand-heldinvasive probe 12 housing at least two light emitters 14, 16, mountedco-linear with one another and with the tip 18 of the probe 12. At leastthee remotely located, one-dimensional light energy sensors 20, 22, and24 are mounted in fixed, spaced relationship to each other and arelocated at known positions and orientations with respect to apredetermined reference coordinate system frame 80. Three light sensors20, 22, and 24 sense the light projected by the individual lightemitters 14, 16 and generate electrical output signals from which arederived the location of the probe 12 and, consequently, the probe tip18, with respect to the fixed coordinate system 80. In addition, thethree sensors 20, 22, 24 could sense and derive the locations of other,optional reference emitters 70, 72 and 74 (FIG. 1B) in the same mannerfor the probe emitters 14 and 16. The role of these reference emittersis to automate the calculation of the transformation matrix between thecoordinate system of the model's image 13 (FIG. 2) of the object and thecoordinate system of the sensors and the object itself 11.

A control unit 30 connected to the moveable probe 12 via a data line 26and coupled to the remotely located sensors 20, 22, and 24 via datalines 28, 32, and 34, respectively, synchronizes the time multiplexingof the two light emitters 14, 15, controls the operation of the sensors20, 22, and 24, and receives data from these sensors as will becompletely described below. A coordinate computer 36, coupled to thecontrol unit 30 by a data line 38, calculates the three-dimensionalspatial positions of the probe 12 and the probe 18, and correlates thosepositions with data from a model 13 of the object 11 which has beenpreviously stored electronically in an electronically accessibledatabase 40 and from correlation information 42. Finally, the computer36 displays a representation 76 of the position of the probe tip 18 withrespect to the computer image 13 of the object 11 on display screen 44(FIG. 2) as will be fully described below.

The probe 12 can be used without the cable 26 coupling it to the controlunit 30 by employing distinctive modulation of the light emitters 14 and16. For example, the pulse durations or frequencies of each can bedifferent. The controller 30, by detecting the pulse duration orfrequency, can determine to which light emitter the sensors 20, 22, and24 are reacting.

The optical mensuration and correlation apparatus 10 of the presentinvention is primarily designed to aid surgeons performing delicateintracranial surgery, and the remaining description is directed to sucha surgical embodiment although many other surgical applications besidescranial surgery are possible. Moreover, the optical mensuration andcorrelation apparatus 10 of this invention may be used for otherpurposes in many various non-medical fields. In the describedembodiment, the physical object 13 of interest is the head or cranium ofa patient, and the model of the cranium is constructed using a series ofparallel internal image slices (of known mutual spatial relationship)such as those obtained by means of computed tomography (CT) or nuclearmagnetic resonance (NMR). These image slices are then digitized, forminga three-dimensional computer model of the patient's cranium which isthen stored in the electronically accessible database 40.

As shown in FIGS. 1A, 1B, 2, and 3 a surgeon places the tip 18 of theprobe 12 at any point on or inside the cranium 11 of the patient. Theposition sensors 20, 22, and 24 detect the locations of the emitters 14,16 attached to the portion of the probe 12 that remains outside thepatient's body. That is, the light produced by the emitters 14, 16 mustbe visible to the sensors 20, 22, and 24. These point emitters 14, 16radiate light through a wide angle so that they are visible at thesensors over a wide range of probe orientations.

The sensors 20, 22, and 24, the control unit 30, and the computer 36determine the three-dimensional location of each emitter 14, 16, andcompute its coordinates in the predetermined coordinate system 80. Thecomputer 36 can then calculate the location of the tip 18 of the probe12 with respect to the predetermined coordinate system 80, according tothe locations of the emitters with respect to the predeterminedcoordinate system 80 and the dimensions of the probe, which dimensionshad been placed into the memory (not shown) of the computer 36beforehand as will be described fully below. Once the computer 36 hascalculated the location of the probe tip 18 with respect to thepredetermined coordinate system 80, the computer 36 then uses therelationship between the model of the cranium stored in the database 40and the coordinate system 80 to calculate the location of the probe tip18 in relation to the model 11. Finally, the computer 36 displays themodel-relative location 76 of the tip 18 on an display screen 44. In asimple form of the preferred embodiment, the computer 36 accomplishesthis display by accessing a CT or NMR image slice 13 stored in thedatabase 40 that is closest to the location of the probe tip 18, andthen superimposes a suitable icon 76 representing the tip 18 on theimage 13 as shown in FIGS. 2 and 3. Thus, the surgeon knows the preciselocation of the probe tip 18 in the patient's cranium relative to theimage data by merely observing the display screen 44. An advanced formof the present invention could derive and display an arbitrary obliquecross-section through the multiple image slices, where the cross-sectionis perpendicular to the probe direction.

The details of the optical mensuration and correlation apparatus 10 ofthe present invention are best understood by reference to FIGS. 1 and 4collectively. Essentially, the probe 12 houses the two light emitters14, 16, which are rigidly attached to the probe 12. Since only twoemitters are used, the emitters 14, 16 must be collinear with the tip 18of the probe 12 so that the computer 36 can determine uniquely theposition of the tip 18 in three dimensions. Moreover, for reasonablemeasurement accuracy, the emitters 14, 16 should be at least as far fromeach other as they are from the tip 18. In any case, the geometricalrelationship of the emitters 14, 16 to each other and to the probe tip18 should be specified to the computer 36 beforehand so that thecomputer 36 can compute the exact location and orientation of the tip 18based on the locations of the individual light emitters 14, 16. The useof three emitters does not require that the probe tip be colinear withthe emitters. Three or more emitters provides full orientationinformation. Although the invention is described as showing only acursor as locating the relative position of the probe tip 18, theinvention can be modified to display a line or shaped graphic toindicate the location of the points of all of the inserted portion ofthe probe. This would entail only the determination of additional pointsalong the probe.

The two light emitters 14, 16 can be high intensity light emittingdiodes (LEDs), which are preferably time multiplexed or strobed bycontrol unit 30 in a predetermined manner such that only one lightemitter LED is "on" or emitting light at any one time as will bedescribed in more detail below. The light emitted from any one of theseemitters 14, 16 is detected by each of the three emitter sensors 20, 22,and 24, which then determine the position of each particular emitter inrelation to the known positions of the detectors 20, 22, and 24 at thetime it is strobed or illuminated.

Each of the one-dimensional sensors 20, 22, and 24 used in the preferredembodiment 10 of the present invention are identical to one another inevery respect. Therefore, for the purpose of giving a detaileddescription of this embodiment, only the sensor 20 is shown anddescribed in detail, since the remaining sensors 22 and 24 areidentical.

In FIG. 4, a one-dimensional sensor 20 comprises a cylindrical lens 46having a longitudinal axis 48 which is orthogonal to the optical axis 50of the sensor 20. A linear photodetector 52, such as a charge coupleddevice (CCD) with several thousand elements or a similar device capableof linear light detection with an elongated aperture 54, is positionedin such a manner that the optical axis 50 passes through the aperture 54and such that the long axis of the aperture 54 is orthogonal to thelongitudinal axis 48 of the lens 46. Light beams 56 from the emitters14, 16 are focused by the cylindrical lens 46 into a real image line 58on the surface 60 of linear photodetector 52.

The photodetector 52 then generates an output 68 (FIG. 5) that isrelated to the position of real image line 58 on the surface 60 ofphotodetector 52, thus characterizing the location of the image itself.That is those elements of the detector 52 illuminated by the real imageline 58 will generate a strong signal while those not illuminated willgenerate none or very small signals. Thus, a graph of image intensity(or signal strength) versus location on the surface of the photodetectorwill resemble the signal peak curve 68 shown in FIG. 5. The"all-emitters-off" (or background), signal level 66 is never quite zerodue to the effects of environmental light, electronic noise, andimperfections in the sensor. In any event, since the image of theilluminated emitter is focused into line 58, only the horizontaldisplacement of emitter 14 from optical axis 50 is measured by detector52, hence the designation "one-dimensional detector."

Thus, a single one-dimensional detector 20 can only locate the plane onwhich an illuminated emitter 14 lies, and the detector 20 cannot, byitself, determine the unique point in space at which illuminated emitter14 is located. To precisely locate the location in space of theilluminated emitter 14 requires three such detectors positioned inspaced relationship to each other since the intersection of three planesare required to define a point.

To locate the position of a particular illuminated one of emitters 14,16, the light sensors 20, 22, and 24 are mounted so that their opticalaxes are not all parallel. In the preferred embodiment, two lightsensors, such as sensors 20, 24 in FIG. 2, are situated so that theirrespective axes 48 (FIG. 4) are in parallel spaced relationship with thethird detector 22 situated between the other two but with its axis 48perpendicular to the axes of the other two. That is, the sensors 20, 22,and 24 should be arranged along a line or arc (FIG. 2), such that eachsensor 20, 22, and 24 is generally equidistant from the center of thevolume in which the measurements are made equally spaced, and all aimedat the center of the measurement volume. If the sensors 20, 22, and 24are arranged along such a horizontal arc, then the middle sensor shouldbe oriented so as to measure the angular elevation of the light emittersas described above. The two outer sensors, therefore, measure thehorizontal angle (azimuth) relative to themselves. Data from the outersensors are used to stereographically calculate both horizontal positionand distance from the sensors as will be described below.

The accuracy of three-dimensional measurements depends on the angleformed between the outer two sensors 20 and 24, where the emitter to bemeasured is at the vertex of the angle. Accuracy improves as that angleapproaches a right angle. All three sensors 20, 22, and 24 must bespaced so that the desired measurement volume is completely within theirfield of view which can be accomplished by making the focal length ofthe lens 46 short enough to provide coverage of the desired field ofview. Alternatively, additional sensors, identical to sensors 20, 22,and 24, could be used either to broaden coverage of the field of view orto enhance measurement resolution.

While this process of detecting the position of a given illuminatedemitter 14, 16 can locate the exact position of the illuminated emitter,it cannot by itself determine the particular orientation and location ofthe probe tip 18 in three-dimensional space. To do so with only twoemitters requires that both emitters 14, 16 be collinear with the probetip 18, as described above. Also, the distances between each emitter 14and 16, (as well as the distances between the emitters 14, 16) and theprobe tip 18 must be known and loaded into the memory of the computer 36before the computer 36 can determine the location of the probe tip 18from the locations of the emitters 14, 16. Consequently, when each ofthe emitters 14, 16 are rapidly turned on in sequence, or strobed, whilethe probe is held relatively stationary, the sensors 20, 22, and 24 candetect the exact position of each emitter in turn. Thus computer 36 candetermine the exact location and orientation of the probe tip 18. Sinceonly one of the emitters 14, 16 is on at any one time, the detectors 20,22, and 24 locate the position of that particular illuminated emitteronly. If the strobe rate, that is, the frequency at which the emitters14, 16 are turned on and off in sequence, is fast enough, the detectors20, 22, and 24 can, for all practical purposes, determine the positionand orientation of the probe tip 18 at any instant in time.

The detectors or sensors 20, 22, and 24 need only distinguish which ofthe light emitters 14, 16 is "on" or illuminated at any one time. In thepreferred embodiment 10 of the present invention, this function isaccomplished by strobing or illuminating each of the emitters 14, 16 insequence, as described above. However, other methods could be used toallow the detectors or sensors 20, 22, and 24 to distinguish therespective pilot light emitters 14, 16 from one another. For example,different colors of light could be used in conjunction with detectorscapable of distinguishing those particular colors or wavelengths oflight. Alternatively, the respective light emitters 14, 16 could bemodulated with a unique "tone" for each emitter. The control unit 30 orcomputer 36 could then be programmed to demodulate the tone, todetermine to which particular emitter 14 or 16 the position signalbelongs. Numerous other methods of distinguishing the light emitters 14and 16 are possible and would be readily apparent to persons havingordinary skill in the art. Therefore, the present invention should notbe regarded as limited to the particular strobing method shown anddescribed herein.

Auto-focusing or multiple-lens optics may be integrated into the sensors20, 22, and 24 to improve the performance of the system. However, thesimple, fixed-focus optics shown and described herein and in FIG. 4 forone sensor provide a good level of performance if the focal length ofthe lenses 46 are kept short and if the working range of the probe 12 isrestricted. Even if the real image of a light emitter 14 or 16 issomewhat out of focus on the photodetector 52, the angular measurementof the image is still usable. A useable measurement for each of thesensors 20, 22, or 24 to generate could be any of the following: (1) theposition of the photodetector element with peak intensity, (2) theintensity-weighted average of all overthreshold elements, or simply (3)the average of the minimum and maximum elements where the intensity isover some threshold as will be completely described below. Thephotodetector 52 should be placed at the focus for the farthest typicaloperating distance of the light emitters. Closer emitters will formslightly de-focused images 58, but they require less precise angularmeasurement for a given distance accuracy. Furthermore, their de-focusedreal images are brighter, which increases the brightness gradient at theedges of the image.

As described so far, the real image 58 of the currently activatedemitter must be significantly brighter than the rest of the lightfalling on the photodetector 52. Otherwise, other lights or reflectivesurfaces in the field of view of the sensors will hinder the detectionof the emitter's real image. Therefore, it is desirable to include inthe apparatus circuitry to subtract the background light focused on thephotodetectors as will be described in detail below. This circuitryenhances use of the invention where the sensors are to detect the lightemitters against relatively bright backgrounds. While the light emittersare all momentarily extinguished, the one-dimensional data from eachsensor are saved in a memory. This could be done in an analog delay lineor by digitally sampling the output signal and storing it in a digitalmemory. Then, as each emitter is turned on sequentially, the saved dataare subtracted from the current data generated by the illuminatedemitter. If the background data are stored digitally, the new data isalso digitized, and the stored background data is digitally subtractedfrom the new data.

A graphical representation of the light intensity of the image, orequivalently, the generated output voltage amplitude for each element inthe row of photodetector elements, is shown in FIG. 5. The graph depictstypical background image intensities 66 with all emitters off, theintensities 68 with one light emitter on, and the element-by-elementdifference 64 between the intensities with the emitter off and thosewith it on. The measurements will likely contain some random electronicnoise, and two consecutive measurements for a given photodetectorelement may differ slightly even where the background is unchanged.Therefore, the differential intensities 65 between two consecutivemeasurements 66, 68 also contain some random electronic noise. However,the two measurements differ substantially only at the location of thelight emitter image, and this difference exceeds the threshold level 62.

The details of the structure and operation of the control unit 30 arebest seen in FIG. 6. Specifically, control unit 30 (FIG. 1) suppliespower to the light emitters 14, 16, and the light sensors 20, 22, and24. A control and synchronization unit 84 and light source sequencer 88time-multiplexes or strobes the emitter lights 14, 16 individually, asdescribed above, so that the position and orientation of the probe tip18 (FIG. 1) can be determined from the signals received from the lightsensors 20, 22, and 24. The angular data signals received from the lightsensors 20, 22, and 24 are converted by an analog-to-digital converter92. Actually, three analog-to-digital converters are used, as shown inFIG. 6, but only one is labeled and described herein for brevity sincethe other two analog-to-digital converters are identical and are used toconvert the signals from the sensors 22 and 24.

The control and synchronization unit 84 also controls three switches, ofwhich switch 93 is typical, which store all digital data received fromthe sensors 20, 22, and 24 when the light emitters 14, 16 are off andstores these data in a background memory 94. Then, when the lightemitters 14, 16 are illuminated in sequence by light source sequencer18, the control and synchronization unit 84 changes the state of switch93 which then redirects the data from the three sensors 20, 22, and 24to a subtraction unit 91. The subtraction unit 91 subtracts thebackground data from the illuminated data, thus resulting in a signalrelatively free from the background noise signal 66 (FIG. 5) since ithas been subtracted from the signal.

As shown in FIG. 6 in conjunction with FIG. 5, a 1-D (one-dimensional)position calculation unit 95 determines the location of the real imageline 58 on the CCD sensor 52 (FIG. 4) by measuring the locations of theedges 67, 69 of the signal blip 68 (FIG. 5) generated by the CCD sensorbased on a predetermined threshold signal level 62. The 1-D positioncalculation unit 95 then averages the distance between the two edges tofind the center of the signal peak as shown in FIG. 5. This method ofdetermining the center of the signal peak is well shown in the art andneed not be described in further detail. Moreover, numerous othermethods of determining the location of the signal peak or its centroidare known in the art and will be obvious to those of ordinary skill inthe art. The method used depends on the signal characteristics of thelight sensor used as well as the characteristics of the lens system usedto focus the light onto the surface of the detector, in addition toother parameters. Those practicing this invention with the variousalternates described herein would have no trouble selecting a signaldetection algorithm best suited to the particular characteristics of thesensors.

Finally, control unit 30 (FIG. 1) transmits the position data to thecomputer 36. That is when the computer 36 is ready to compute thecurrent location of the illuminated emitter 14 or 16 on the probe 12,the latest angular data from all sensors 20, 22, and 24 are provided foranalyzation. If the sensors generate data faster than the control unit30 can process them, the unprocessed angular data are simply discarded.

The operation of the computer 36 is most advantageously set forth inFIG. 7. The computer 36 calculates one-dimensional positions for eachlight emitter 14, 16, based on the location of the signal peak from eachrespective sensor 20, 22, and 24. These one-dimensional position signalsare then used to determine the three-dimensional spatial coordinates ofthe emitters 14, 16, and thus for the probe 12 relative to thepredetermined coordinate system 80 by coordinate transformation methodswhich are well-known in the art. The output signals from the computer 36can be in any form desired by the operator or required by theapplication system, such as XYZ coordinate triples based upon somepredetermined stationary rectangular coordinate system 80.

FIG. 8 and the following paragraphs describe in detail how the positionof a single light emitter 14, 16 is determined from the data returnedfrom the sensors 20, 22, and 24. The following description applies tothese three sensors 20, 22, and 24 only. If there are more than threesuch sensors, the calculation can be performed using any three of thesensors. Furthermore, if more than three sensors are used, the averageof the points calculated from all combinations of three sensors could beused to increase accuracy. Another option is to use the point calculatedfrom the three sensors closest to the light emitters 14, 16. Thefollowing parameters are considered to be known:

DO[i], one endpoint of each linear photodetector i;

Dl[i], the other endpoint of linear photodetector i;

LO[i], one endpoint of the axis of each lens i; and

Ll[i], the other endpoint of the axis of lens i.

T[i], a parametric value between 0 and 1 indicating where the peak orcenter of the line image of the emitter intersects the line segmentbetween DO[i] and Dl[i] is

The coordinates of point S are to be calculated, where S is the locationof the light emitter.

For a CCD photodetector array, T[i] is the index of the element on whichthe center or peak of the image falls divided by the number of elementson the photodetector array, where i=1,2,3, . . . n, which is a number ofsensors being used.

The three-dimensional coordinates of the above points are all referencedto a predetermined coordinate system 80. The cylindrical lens and linearphotodetector do not directly measure an angle A of the light emitterabout its lens axis; rather, they measure a value T[i] linearly relatedto the tangent of that angle by

    tan(A)=C * (2*T[i]-1),

where C is a constant of proportionality that is related to anddetermined empirically by, the dimensions of a particular system.

The three-dimensional location where the image line intersects thelinear photodetector is

    D[i]=(1-T[i])*DO[i]+(T[i])*Dl[i]

If the lens is ideal, then S also lies in plane P[i]. In reality, thepoint D[i] might have to be computed by a non-linear function F(t) thatcorrects for non-linear aberrations of the lens or the photodetector:

    D[i]=(1-F(T[i]))*DO[i]+(F(T[i]))*Dl[i]

Function F(t) could be a polynomial in variable T, or it could be avalue interpolated from an empirically determined table.

P[i] is a unique plane determined by the three points D[i], LO[i], andLl[i], which are never collinear. S is the point of intersection of theplanes P[1], P[2], and P[3] determined by sensors 20, 22, and 24. S is aunique point if at least two sensor lens axes are not parallel and if notwo lens axes are collinear. The intersection point is found by findingthe common solution S of the three equations defining the planes P[i].Once the location S of each of the probe's light emitters is computed,the location of the probe's tip 18 can be calculated. The method ofmaking such a determination is well known using the teaching of analyticgeometry and matrix manipulations.

If M is a linear transformation matrix describing the relationshipbetween a point R in the model space and a point S in the sensor space,then

    R*M=S.

If M' is the inverse of M and if S is a point in the sensor space, thenthe point in the model space corresponding to S is

    S*M'=R.

The preliminary steps required before practicing the method of theinvention are now described. Then, after fully describing thesepreliminary steps, the detailed steps of the method of the opticalmensuration and correlation apparatus are described.

Use of the invention takes place in three phases: the imaging phase; thecorrelation phase; and the normal operational phase. The imaging phaseprecedes the normal operation of the present invention. During theimaging phase, the body of the object of interest is used to build athree-dimensional geometrical model. In the preceding description, theobject was the head of a human intracranial surgical patient because theinvention is advantageously used in stereotactic neurosurgery.Accordingly, the three-dimensional model may comprise digital data froma series of internal cross-sectional images obtained from computedtomography (CT), magnetic resonance (MR), ultrasound, or some otherdiagnostic medical scanner. In any case, the image data stored in asuitable, electronic memory 40 which can be accessed later by thecomputer 36. The data is considered to be stored as a series of paralleltwo-dimensional rectangular arrays of picture elements (pixels), eachpixel being an integer representing relative density. If the object isrelatively rigid like a human head, this three-dimensional model may becreated before the correlation and operational phases of the inventionand possibly at another location.

Also, during the imaging phase, at least three non-collinear referencepoints 71, 73, and 75 (FIGS. 1A, 2 and 3), must be identified relativeto the object 11. These may be represented by ink spots, tatoos,radiopaque beads, well-defined rigid anatomical landmarks, locations ona stereotactic frame, sterile pins temporarily inserted into rigidtissue or bone of a surgical patient, or some other reference means. Thecoordinates of these reference points are measured and recorded relativeto the coordinate system of the imaging device. One way to accomplishthis is to capture the reference points as part of the three dimensionalmodel itself. For example, radiopaque pins could be placed within theimage planes of diagnostic CT slices; the pin locations, if notautomatically detectable because of their high density, can beidentified interactively by the surgeon using a cursor on the computerdisplay of the CT slices. See FIG. 3.

The correlation mode immediately precedes the normal operational phaseof the present invention and must take place in the operating room.During the correlation phase, the invention accesses the data of thethree-dimensional geometrical model of the patient, including thereference point coordinates which were recorded earlier. Next thesurgeon places the tip of the probe 18 at each of the reference points71, 73, and 75 on the patient, in turn, as directed by the computerprogram. This establishes a relationship between the locations of thesereference points in the model space and their current physical locationsin the predetermined coordinate system 80. In turn, this establishes alinear mathematical relationship between all points the model andcoordinate system 80. Thereafter, if the patient is ever moved relativeto the sensors, a new relationship must be defined by again digitizingthe reference points 71, 73, and 75 within the coordinate system 80.That is, the correlation phase must be repeated. Thereafter, a surgeoncan relate any locations of interest on the diagnostic images with thecorresponding physical locations on his patient during an operation, andvice versa. These could be locations accessible to the probe tip 18 butnot necessarily directly visible to the surgeon.

Having described the function and purpose of the preliminary steps, thedetailed method of the present invention is more easily understood. Asshown in FIG. 7, the position data 21 of the probe emitters 14, 16generated by the sensors 20, 22, 24 and control unit 30 are convertedinto three-dimensional coordinates relative to the predeterminedcoordinate space of the sensors 20, 22, 24. Using dimensional parametersdescribing the relationship among the probe emitters 14, 16 and probetip 18, the computer 36 determines the coordinates of the probe tip 22in a step 39. During the correlation phase, the probe tip 22 is placedat each of the reference points 71, 73, and 75. Their coordinates in thesensor space along with their coordinates 46 in the image spacedetermine a unique linear transformation relating the two spaces in astep 45. This is a known calculation in analytic geometry and matrixmathematics.

A more automated method of determining the locations of the referencepoints 71, 73, and 75 is to place other emitters 70, 72 and 74 (FIG. 1B)at those reference points and use the sensors 20, 22, 24 to determinetheir locations relative to the predetermined coordinate system 80 ofthe sensors. In that case, the correlation phase could be automaticallyinitiated by the computer 36 except for the manual attachment of theadditional emitters 70, 72, 74 at the reference points 71, 73, 75 justbefore using the device. In fact, the correlation phase could befrequently but briefly repeated from time to time (continuously orinitiated by the operator), interspersed in the operational phase forthe purpose of recalculating the linear transformations M and M' shouldthe object (such as a surgical patient) move relative to the sensors 20,22, 24.

During normal operation, the tip coordinates are transformed in a step44 using the transformation computed in the step 45. The new transformedcoordinates, relative to the image space, are used to determine theplane of some two-dimensional cross-section through thethree-dimensional image model 41 accessible in the accessible memory 43.The simplest method is simply to choose the existing diagnostic imageplane located closest to the probe tip's coordinates relative to themodel space.

A more advanced method requires synthesizing a cross-sectional image atsome other angle through the model using the data from multiple imageslices.

In any case, a step 47 transforms the two-dimensional cross-sectionalslice to a screen image and places a cursor on it to mark the locationof the probe tip in the image space. Scaling and viewing parametersdetermine how the image is displayed. Because the surgeon cannotsimultaneously view the patient (object) and the computer displayscreen, the step 47 would only be executed when a button on the probe ispressed, freezing the image and the position of the probe tip marker atthat instant.

In a more complex variation of this invention, the computer system couldgenerate the displayed image slice at an arbitrary angle, for example,in the plane perpendicular to the direction the probe is pointing. Insimpler cases, the computer would simply display any one or moreconvenient image slices through the location of the probe tip. Forexample, the displayed slice might simply be the original CT sliceclosest to that location. In any case, the computer would then displayon the image a cursor at the current position of the probe tip.

An alternative means to record the location of the reference points inthe coordinate space of the imaging apparatus during the imaging phaseemploys an additional, separate instance of the three-dimensionalposition mensuration probe, sensors, control unit, and computer of thepresent invention. The additional sensors would be permanently attacheddirectly on the imaging apparatus. The additional probe would measurethe location of the reference points at the time of imaging, and theadditional control unit and computer would determine and record theirlocations relative to the coordinate system of the imaging apparatus.The advantage of this approach is that the landmarks or reference pinsneed not be within the limited cross-sectional slices visible to theimaging device.

As an alternative to true three-dimensional images, standard x-rayradiographs from several distinct directions can be used to construct acrude model in lieu of the imaging phase described above. Radiographsfrom two or more directions would be digitally scanned, and fournon-coplanar reference points on them would be identified with a cursoror light pen. In a correlation phase similar to that described above,these four points on the patient would be digitized just prior tosurgery. Then, during surgery, the location of the probe tip would beprojected onto digitized computer images of the two-dimensionalradiographs where the projection is uniquely defined by mapping thereference point coordinates from the model space to the sensor spaces.

Lastly, a videotape recording of the computer screen (as well as thedirect view of the surgeon and patient) could help document theperformance of the procedure. Light emitters could be present on morethan one standard surgical tool such as the microscope, scalpel,forceps, and cauterizer, each of which would in effect be a probe.

The method and apparatus of the optical mensuration and correlationapparatus 10 of the present invention has been completely described.While some of the obvious and numerous modifications and equivalentshave been described herein, still other modifications and changes willreadily occur to those skilled in the art. For instance, the preferredembodiment uses visible light, since human operators can readily observeif the light sources are operative or whether they are causingtroublesome reflections. Clearly, other wavelengths of electromagneticradiation could be used without departing from the spirit and scope ofthe invention. Infrared light would have the advantage of notdistracting the surgeon with flashing lights. Other modifications to thedetector optics and lenses are possible which would alter the imagecharacteristics on the sensors. For example, toroidal lenses could beused which are longitudinally curved along an arc with a radius equal tothe focal length of the lens. Similarly, the surfaces of thephotodetectors could also be curved, thus allowing the images of distantlight sources to remain in sharp focus, regardless of their positions.Various measurements of the detector outputs are also possible. Forexample, the angle of peak intensity, the intensity-weighted average, orthe average of the minimum and maximum angles where the intensity isover some predetermined threshold value could be used. Finally, numerousenhancements of the digital data are possible by programming thecomputer to make the appropriate enhancements as would be obvious tothose persons having ordinary skill in the art.

The foregoing is illustrative of the principles of the invention. Sincenumerous modifications and changes will readily occur to those skilledin the art given the teachings of the invention, it is not desired tolimit the invention to the exact construction and operation shown anddescribed. Accordingly, all suitable modifications and equivalents thatmay be resorted to in light of disclosure of the invention fall withinthe scope of the invention as defined by the following claims.

I claim:
 1. Apparatus for determining the position and orientation of aninvasive portion of a probe inside a three-dimensional body, wherein theprobe includes an external portion that remains outside the body,comprising:means defining a coordinate system wherein the threedimensional body is adapted to be disposed within said coordinationsystem; means defining an electronically displayable model of said body;a probe comprising a portion adapted to be external to said threedimensional body and an invasive portion adapted to be internal to saidthree dimensional body; at least two probe light emitters mounted inspaced relationship on the external portion of the probe each forprojecting a probe light ray; at least three light sensors, in knownlocations within said defined coordinate system, remotely located fromthe probe for detecting at least two probe light rays; means to causeprobe light rays to pass between said probe light emitters and said atleast three light sensors which are non-linear with respect to eachother; means to measure angles between said probe light beams to atleast three of said light sensors; and means coupled to said at leastthree light sensors for converting said angles to current locations ofsaid probe light emitters and, from said locations, determining theposition and orientation of the probe relative to said definedcoordinate system and for deducing the position and orientation of theinvasive portion of the probe by correlating the position andorientation of the probe relative to the defined coordinate system. 2.The apparatus of claim 1 further comprising display means coupled tosaid location determining means for indicating the position andorientation of the invasive portion of the probe by displaying arepresentation of the invasive portion of the probe tip in spacialrelationship to a display of said model of the body.
 3. The apparatus ofclaim 2 wherein said at least three light sensors comprises at leastthree one-dimensional light sensors in spaced relationship for sensinglight rays from said probe light emitters.
 4. The apparatus of claim 3wherein each said one-dimensional light sensors comprises:a linearphotodetector; and a lens positioned between said linear photodetectorand said probe light emitters for focusing light rays from said probelight emitters onto said linear photodetector.
 5. The apparatus of claim4 wherein each of said probe light emitters is strobed off and on bysaid computing means in a predetermined manner so that only one of saidprobe light emitters is illuminated at any one time.
 6. The apparatus ofclaim 5 wherein each said lens of each said one-dimensional light sensorcomprises a cylindrical lens.
 7. The apparatus of claim 6 wherein saiddisplay means is a video monitor.